docs: def import_utterances(self, utterances):
          """
          Add the given utterances/utterance to the corpus.
          If any of the given utterance-ids already exists, a suffix is appended so it is unique.
  
          Args:
              utterances (list): Either a list of or a single :py:class:`audiomate.tracks.Utterance`.
  
          Returns:
              dict: A dictionary containing idx mappings (old-utterance-idx/utterance-instance).
                    If a utterance is imported, whose id already exists this mapping can be used to
                    check the new id.
          """
  
          if isinstance(utterances, tracks.Utterance):
              utterances = [utterances]
  
          idx_mapping = {}
  
          for utterance in utterances:
              idx_mapping[utterance.idx] = utterance
  
              # Check if there is a track with the given idx
              if utterance.track not in self._tracks.values():
                  raise ValueError('Track with id {} is not in the corpus.'.format(utterance.track.idx, utterance.idx))
  
              # Check if there is a issuer with the given idx
              if utterance.issuer is not None and utterance.issuer not in self._issuers.values():
                  raise ValueError('No issuer in corpus with id {} to add utterance {}.'.format(
                      utterance.issuer.idx, utterance.idx))
  
              # Add index to idx if already existing
              if utterance.idx in self._utterances.keys():
                  utterance.idx = naming.index_name_if_in_list(utterance.idx, self._utterances.keys())
  
              self._utterances[utterance.idx] = utterance
  
          return idx_mapping